Electronic components having a small equivalent series resistance (ESR) and being excellent in high frequency characteristics have recently required as digital equipments have been developed. Solid electrolytic capacitors employing solid electrolyte, such as manganese dioxide, polyaniline, polypyrrole, or polythiophene are put in the market as an electronic component. A capacitor including a roll type functional element having a rolled electrode foil and a capacitor including a stacked type functional element having electrode foils stacked in a thickness direction are known as the solid electrolytic capacitors. The roll type element can have a larger capacitance due to its structure and can be more efficiently produced than the stacked type element.
FIG. 34 is a cross-sectional view of solid electrolytic capacitor 601, a conventional electronic component including a roll type functional element disclosed in JP6-45201A. Solid electrolytic capacitor 601 includes roll type capacitor element 101, a pair of lead wires 102 drawn out from capacitor element 101, and outer package resin 103 that covers capacitor element 101. Capacitor element 101 includes a positive electrode foil having a dielectric oxide layer on a surface roughened by an etching process, a negative electrode foil, and a solid electrolyte layer produced on a surface of the positive electrode foil. A separator is provided between the positive electrode foil and the negative electrode foil. The positive electrode foil, the negative electrode foil, and the separator are rolled to form roll type capacitor element 101. Respective one ends of the pair of lead wires 102 has one end rolled in and connected to the positive electrode foil and the negative electrode foil of capacitor element 101. Respective other ends are drawn out from the electrode foils of capacitor element 101. Outer package resin 103 covers capacitor element 101 to expose respective portions of lead wires 102 to outside. The solid electrolyte layer includes a polyaniline film formed on the surface of the positive electrode foil, and a polypyrrole film formed on the polyaniline film. The polypyrrole film is formed by chemical polymerization.
In solid electrolytic capacitor 601, fine etching pits formed in the surface of the positive electrode foil by etching increases the surface area of the surface at a large magnification. The solid electrolyte layer is uniformly and densely formed within the fine etching pits, hence providing solid electrolytic capacitor 601 with excellent electrical characteristics, such as a large capacitance, as well as a small size and a low profile.
Solid electrolytic capacitor 601 including conventional roll type capacitor element 101 is prevented from having a smaller size and a lower profile while maintaining the electrical characteristics, such as large capacitance.
Roll type capacitor element 101 generally has a cylindrical shape. An electronic component, such as solid electrolytic capacitor 601, occupies substantially a space having a rectangular parallelepiped shape when mounted on a circuit board. Therefore, a dead space is formed at four corners of the space when circular columnar shaped capacitor element 101 is accommodated in the space having the rectangular parallelepiped shape. In order to reduce the size and height of conventional solid electrolytic capacitor 601, the width, length, and thickness of the electrode foils of capacitor element 101 need to be further reduced as capacitor element 101 cannot effectively occupy the dead space. The capacitance per unit area of the electrode foil can be increased by raising the magnification of increase of the surface area by etching even while the width, length, and thickness of the electrode foil are reduced. However, the etching pits becomes finer if the surface area is increased by etching, and accordingly, prevents the solid electrolyte from covering the surface of the electrode foil, hence providing the capacitor with a large resistance. The finer pits reduces the strength of the electrode foil, hence preventing control in production processes, such as lead wire connection and electrode foil rolling. Thus, conventional solid electrolytic capacitor 601, upon having a smaller size and a smaller height, adversely affects electrical characteristics, such as a small capacitance and a large ESR.